x86: fix clang .macro retention check

There were two problems here: The first closing parentheses got parsed
by make to end the $(call invocation, and the escaping of the quotes
wasn't right either, as there's nowhere they would get un-escaped.

Furthermore there appears to be a puzzling problem with \n getting
expanded to an actual newline too early in some environments. Convert
these to semicolons at the same time.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Tested-by: Roger Pau Monné <roger.pau@citrix.com> [On FreeBSD and Debian 9.5]
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>

Config.mk

@@ -6,6 +6,8 @@ endif
 
 # Convenient variables
 comma   := ,
+open    := (
+close   := )
 squote  := '
 #' Balancing squote, to help syntax highlighting
 empty   :=

xen/arch/x86/Rules.mk

@@ -81,6 +81,6 @@ $(call as-option-add,CFLAGS,CC,".include \"asm/indirect_thunk_asm.h\"",,\
 # Check whether clang keeps .macro-s between asm()-s:
 # https://bugs.llvm.org/show_bug.cgi?id=36110
 $(call as-option-add,CFLAGS,CC,\
-                     ".macro FOO\n.endm\"); asm volatile (\".macro FOO\n.endm",\
+                     ".macro FOO;.endm"$$(close); asm volatile $$(open)".macro FOO;.endm",\
                      -no-integrated-as)
 endif